1. Field of the Invention
The present invention relates to a disconnection detecting device, in particular, a disconnection device to detect a disconnection of electric paths connecting each both ends of a plurality of series-connected unit cells with a voltage measuring device for measuring a voltage between both ends of the unit cells.
2. Description of the Related Art
Recently, a hybrid vehicle (hereinafter referred to as “HEV”) running with an engine and an electric motor in combination has become widely used. This HEV includes a low-voltage battery about 12 V for starting the engine, and a high voltage battery for driving the electric motor. This high voltage battery is made of series-connected unit cells. The unit cell is a secondary battery such as nickel hydride battery or lithium battery.
After repeating charge and discharge cycles of the high voltage battery, a voltage across each unit cell, namely, a state of charge (SOC) is varied. With regard to the charge and discharge of the battery, in view of endurance of the battery and safety ensuring, when the unit cell having the highest SOC (or a voltage between both ends) reaches an upper limit, the charge is stopped, and when the unit cell having the lowest SOC (or a voltage between both ends) reaches a lower limit, the discharge is stopped. Accordingly, when the SOCs of the unit cells are varied, an enabled capacity of the battery is substantially reduced. Therefore, in the HEV, so called assistance and regeneration of the battery become insufficient, and a moving performance and mileage are reduced. Consequently, for equalizing the SOCs of the unit cells, the voltages across the unit cells should be measured.
Conventionally, both ends of each unit cell are connected to a voltage measuring IC as a voltage measuring device, and the voltage measuring IC measures the voltage between both ends of each unit cell. However, when an electric path connecting the unit cells with the voltage measuring IC is disconnected, voltage potential at the electric path side which is disconnected is unstable due to an influence of an internal impedance of the voltage measuring IC. Therefore, there is a problem that the voltage between both ends of the unit cell cannot be correctly measured.
So, a disconnection detecting device to detect the disconnection when a voltage difference between the voltage between both ends of the unit cell when a bypass resistor is turned on and the voltage between both ends of the unit cell when the bypass resistor is turned off is large, where a bypass circuit composed of the bypass resistor and a switching device to turn on/off the bypass resistor is connected in parallel to the unit cell is proposed (Patent Document 1).
However, in the above described disconnection detecting device, it is necessary that the bypass resistor is arranged parallel to the unit cell. Therefore, a bypass resistor is needed other than a resistor for a noise filter connected between both ends of the unit cell and the voltage measuring IC, and a discharge resistor for equalizing the unit cell. Therefore, there is a problem that the number of electric components is increased. Further, when the resistance value of any one of the resistor for noise filter and the bypass resistor is varied, the measuring voltage is largely varied. Therefore, there is a problem that the variation of the resistance value causes the false detection.
[Patent Document 1] Japanese Patent No. 3839397
Accordingly, an object of the present invention is to provide a disconnection detecting device for detecting a disconnection of an electric path at a low price.